Projection exposure apparatus for transferring mask pattern onto photosensitive substrate

ABSTRACT

A projection exposure apparatus detects positions at the measurement points (P 1 -P 5 ) in the Z-direction on the shot area of the wafer W, and obtains the distribution of the irregularity of the shot area based on the detected result and the pre-known process structure data. For example, when the pattern leaving the narrowest line width is exposed in the pattern area ( 40 B), the pattern area ( 40 B) is made as a focusing reference plane and the difference in level (Z A −Z B ) of another area of which reference is pattern area ( 40 B) is added to the level of the best image plane ( 42 ) as an offset value. The pattern area ( 40 B) is focused to the best image plane ( 42 ) by fitting image plane ( 42 A) to the exposure surface.

This application is a continuation of U.S. patent application Ser. No.09/323,042 filed Jun. 1, 1999, now U.S. Pat. No. 6,195,154, which is acontinuation of U.S. patent application Ser. No 08/823,678 filed Mar.25, 1997, now abandoned, which is a continuation of U.S. patentapplication Ser. No. 08/436,557 filed May 8, 1995, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a projection exposure apparatus fortransferring a mask pattern such as a circuit pattern on aphotosensitive substrate, more particularly, it relates to a projectionexposure apparatus provided with a focus detection device for focusingon the photosensitive substrate.

2. Related Background Arts

In the conventional projection exposure apparatus, when a pattern of areticle (a photomask or the like) is projected onto a photosensitivesubstrate (a wafer, a glass plate or the like on which a photoresistlayer is coated) through a projection optical system with a highresolution to expose the substrate, an exposure surface on thephotosensitive substrate must be accurately coincided with the imageplane of the pattern on the reticle, that is focusing of the pattern ofthe reticle is necessary. Recently, the focal depth of the projectionoptical system tends to become narrow. However, a dept of only about±0.7 μm can be obtained under the present state though i line having thewavelength of 365 nm is used as an illumination light for exposure.Moreover, a projection visual field of the projection optical systemtends to expand year by year, so that it is desirable that a largestfocal depth is obtained in all of a board exposure visual field (forexample, a square having side of 22 mm).

To achieve satisfactory focusing across the broad exposure visual field,a better flatness of a partial area (shot area) on the photosensitivesubstrate within the exposure visual field and a better flatness of theimage plane (that is, the curvature and the inclination of the imageplane are small) must be obtained. The curvature and the inclination ofthe image plane are mainly dependent on the optical performance of theprojection optical system, in addition, they are sometimes dependent onthe flatness of the reticle and/or the parallelism between the reticleand the substrate. On the other hand, the flatness of the partial areaon the photosensitive substrate, that is the flatness of everyprojection exposure area (shot area) differs from substrate tosubstrate. However, the surface in the shot area on the photosensitivesubstrate can be set in parallel with the image plane by inclining aholder for holding the photosensitive substrate by a small angle.

The methods for performing focusing under consideration of the surfaceinclination in a one shot area on the photosensitive substrate aredisclosed in Japanese Patent Application Laid-Open No. Sho 58-113706(U.S. Pat. No. 4,558,979) and Japanese Patent Application Laid-Open No.Sho 55-1348 (U.S. Pat. No. 4,383,757). Particularly, U.S. Pat. No.4,383,757 discloses the technique, wherein spots of the light beam areprojected at four points on a photosensitive substrate through aprojection optical system and then a spot image formed by the reflectedlight is photo-detected to carry out focusing of the photosensitivesubstrate and correction of the inclination thereof (leveling).

However, since recent semiconductor devices are manufactured bysuperposing many complex structure patterns on a substrate, the flatnessof the exposure surface on the photosensitive substrate becomes worse.Therefore, a technique has been developed wherein an irregular conditionin the shot area on the photosensitive substrate is measured and then anaverage surface in the shot area is focused onto the image plane by theprojection optical system based on the measured result. For example,Japanese Patent Application Laid-Open No. Hei 2-198130 (U.S. Pat. No.5,124,562) discloses a surface position detecting method, whereinphotosensitive substrate is fixed in the direction along the opticalaxis of the projective optical system but moved in the directionperpendicular to the optical axis, the positions (focus position) in thedirection along the optical axis of the projective optical system aremeasured at a plurality of measurement points in the shot area on thephotosensitive substrate, and then the average of the measured resultsis obtained, whereby the offset value of the focus position, which isbased on the differences in structure and/or position of the patterns inthe shot area, is obtained. In this method, the average focus positionis measured in consideration of the irregularities in the shot area byadding the offset value to the measured result of the focus position ateach shot area, for example, at the center measurement point.

As described above, in the conventional projection exposure apparatus,the offset value of the focus position is obtained by averaging thefocus positions measured at a plurality of specific measurement points.However, in practice, the irregular conditions of the exposure surfacein every shot area on the photosensitive substrate vary in accordancewith the process construction (such as arrangements and differences inlevel of patterns), so that, the average surface shape at every shotarea can not be obtained accurately only by averaging the focus positionat a plurality of specific measurement points. Therefore, if thearrangement, the differences in level and the like of the pattern inevery shot area on the photosensitive substrate change, there is adisadvantage that the average surface in every shot area can sometimesnot be placed within a range of the focal depth with respect to theimage plane of the projection optical system.

Moreover, it is difficult to conform the area to the image plane by theconventional method even when the average surface in every shot does notconform to the image plane, and, an area within the shot area, in whicha pattern with the narrowest line width is exposed, is mainly conformedto the image plane.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a projectionexposure apparatus, wherein each shot area can be fitted to an imageplane by a projection optical system in an optimum condition and beexposed, irrespective of an irregularity in every shot area on aphotosensitive substrate.

It is another object of the present invention to provide a projectionexposure apparatus, wherein an offset value for fitting a focusingreference plane on a substrate to an image plane by a projection opticalsystem is obtained based on detected signals corresponding to horizontaldisplacement values of each of a plurality of images which are re-imagedby a light-receiving optical system, and a process construction of anexposure surface on the substrate, whereby a shot area can be fitted toan image plane in an optimum condition for exposure.

It is a further object of the present invention to provide a projectionexposure apparatus, wherein an offset value for fitting a focusingreference plane on a substrate to an image plane by a projection opticalsystem is obtained based on detected signals of a photoelectricdetecting device at a plurality of measurement points which aredistributed over the entire surface and a process construction of anexposure surface on the substrate, whereby a shot area can be fitted toan imaged surface in an optimum state for exposure.

It is a further object of the present invention to provide a projectionexposure apparatus, wherein a wave-length sensitive characteristics of alight beam, which is used when an image of a pattern for detecting afocal point is projected in an exposure area, is uniform in an opticalpath of an optical system, whereby a shot area can be fitted to an imagesurface in an optimum state for exposure.

A projection exposure apparatus according to the present inventioncomprises a projection optical system for projecting a mask pattern ontoa photosensitive substrate; a substrate stage for holding the substrateand for positioning the substrate in a plane perpendicular to an opticalaxis of the projection optical system; a focus leveling stage foradjusting an inclination of the substrate and a level of the substratein a direction along the optical axis of said projection optical system;a focusing projection optical system for projecting an image of a focusdetecting pattern onto a plurality of measurement points in an exposurearea by the projection optical system obliquely relative to the opticalaxis of said projection optical system using a light to which thesubstrate is not sensitive; a light-receiving optical system forcollecting the reflected light from the plurality of measurement pointsto re-image the focus detecting pattern on said plurality of measurementpoints; a plurality of photoelectric detecting devices for generatingdetected signals corresponding to horizontal displacement offset valuesof a plurality of images which are re-imaged by the light-receivingoptical system; and, a control device for obtaining separately for eachof said plurality of measurement points an offset value for fitting afocusing reference plane on the substrate to an image plane by saidprojection optical system with an operating device based on respectivedetection signals of the photoelectric detecting device corresponding tothe plurality of measurement points and a process structure (anarrangement of patterns or difference in level) of an exposure surfaceon said substrate, and for controlling an operation of the focusleveling stage using said offset value.

In this case, it is desirable that the substrate stage is driven to movethe substrate with the image of the focus detecting pattern beingprojected from the focusing projection optical system onto the exposurearea (SA) of the projection optical system, thereby obtaining detectedsignals by the photoelectric detecting devise, which correspond to saidplurality of measurement points distributed over the entire surface inthe exposure area (SA), and the arithmetic unit obtains separately anoffset value for fitting a focusing reference plane (40B) on saidsubstrate to an image plane of the projection optical system for each ofthe plurality of measurement points, based on detected signals by thephotoelectric detecting device at a plurality of measurement pointsdistributed over the entire surface and the process structure of theexposure surface of the substrate.

It is desirable that a light beam used while the image of thefocus-detecting pattern is projected from the focusing projectionoptical system onto the exposure area of the projection optical systemhas a bandwidth more than 100 nm.

Further, it is desirable that an optical filter, for making uniformwavelength sensitive characteristics of a light beam which is used whenthe image of the focus detecting pattern is projected from the focusingprojective optical system onto the exposure area of the projectionoptical system, is located in an optical path from the focusingprojection optical system to the plurality of photoelectric detectingdevices.

Moreover, the arithmetic unit, preferably, corrects a desirable valuecorresponding to a level of the image plane of the projection opticalsystem based on the offset values which are obtained separately at eachof the plural measurement points.

According to the present invention, as shown in FIG. 8(a), the images ofthe focus-detecting pattern are projected onto the plurality ofmeasurement points (P1-P5) within the exposure area of the projectionoptical system on the substrate and re-imaged by the light-receivingsystem, and the detected signals (FSa-FSe) corresponding to thehorizontal displacement values of the re-imaged image are outputted fromthe photoelectrical detecting device (such as picture elements of thearray sensor in FIG. 7). In an oblique incident type, the horizontaldisplacement value of the re-imaged image is almost proportional to theposition (the focus position) of the corresponding measurement point inthe direction along the optical axis of the projection optical system.Therefore, the focus positions (Z₁-Z₅) of the corresponding measurementpoints can be calculated from the detected signals.

However, in practice, as shown in FIG. 8(a), there is a case that anirregular pattern is formed by the exposure process and the like carriedout before then. When there are such irregularities, if a surface onwhich the pattern with the narrowest line width (for example, it isknown that the surface sinks relative to the peripheral portion) ispresumed to be the surface (40B), it is desirable that the surface (40B)is coincided with the image plane. In this case, it is understood thatif the value (focus position) of the detected signal measured at themeasurement point (P3) is minimum, for example, the measurement point(P3) is on the surface (40B). Thus, it is presumed that the surface(40B) is a focusing reference plane on the substrate, and, the detectedsignals corresponding to difference in level (Z_(A)−Z_(B)) between thereference plane (40B) and another exposure surface (40A, 40C) andobtained based on the process structure date are offset values at othermeasurement points (P1, P2, P4, P5). The offset value is 0 at themeasurement point (P3).

Then, if focusing and leveling are performed based on the value that theoffset value is subtracted from the actual, detected signal, thereference plane (40B) is focused on the image plane as shown in FIG.8(b).

When the plurality of measurement points (P1-P5) are arrangedsubstantially on the diagonal line of the exposure area (SA), forexample, as shown in FIG. 5, the detected signals at the measurementpoints distributed over the entire surface of the exposure area (SA) canbe obtained by scanning the substrate in a predetermined direction(X-direction) relative to the projected image of the focus-detectingpattern on the measurement points (P1-P5). Therefore, though complexirregularities are distributed over the entire surface, a predeterminedportion of the surface (such as the area exposed by the pattern with thenarrowest line width) in which the irregularities are distributed isused as the reference plane, and a detection signal corresponding to thedifference in level between the reference plane and other portions isused as an offset value at every measurement point. With thisarrangement, the reference surface can be focused.

Next , according to another example of the procedure for performingfocusing and leveling, for example, in FIG. 8(a), first, an offset valuecorresponding to the difference in level (Z_(A)−Z₅) between thereference plane (40B) and other surfaces (40A, 40C) is obtained based onthe detected signal by the photoelectric detecting device and theprocess structure, and then the offset value is added to the level ofthe detected signal corresponding to the level of the image plane. Thesurface corresponding to the above added result is shown as adotted-line surface (42A). Thus, with the least square method, the levelof the substrate is controlled so as to minimize the difference betweenthe detected signals at the respective measurement points (P1-P5) andthe detected signal of the surface (42A), whereby as shown in FIG. 8(b),the reference plane (40B) is focused at an actual image plane.

When the light beam, which is used while the image of thefocus-detecting pattern is projected from the focusing projectionoptical system onto the exposure area (SA) by the projective opticalsystem, has a bandwidth more than 100 nm, the adverse effect and thelike by a thin film interference in the photosensitive material(photoresist or the like) on the substrate can be reduced.

Further, when an optical filter, for making uniform the wavelengthsensitive characteristics of the light beam used when the image of thepattern for level-detecting is projected from the focusing projectionoptical system onto the exposure area by the projection optical system,is placed in the optical path from the projection optical system to theplurality of photoelectric detecting devices, even if the lightintensity distribution of the focus detecting illumination light forevery wavelength is uniform such as, for example, in FIG. 15(a), thetransmittance distribution of the optical filter is set, for example, asshown in FIG. 15(b), so that the optical filter has characteristicsreverse to the light intensity distribution of the illumination light.With this arrangement, the wavelength characteristics of the detectedsignal obtained from the photoelectric detecting device becomes flat asshown in FIG. 15(d). Therefore, without large effect by the signalhaving a specific wavelength, the level can be detected accurately.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a structural view including a partial sectional view showing adetecting mechanism for a best image plane of a projection opticalsystem in one embodiment of a projection exposure apparatus according tothe present invention;

FIG. 2(a) is an enlarged plan view showing an arrangement of a marks ona reference mark plate FM, and FIG. 2(b) is an enlarged view showing arelation between an image, which is re-imaged on the reference markplate FM, and the marks;

FIGS. 3(a) and 3(b) are views showing respective changes in level ofsignals KS-outputted from the detecting mechanism in FIG. 1;

FIG. 4 is a structural view showing an optical system and a controlsystem of a multipoint AF system in the embodiment;

FIG. 5 is a view showing a slit image projected in an exposure field ofa projection optical system PL by means of the multipoint AF system inFIG. 4;

FIG. 6 is a disassembled perspective view showing a relation between aslit plate 14 and an array sensor 15 in FIG. 4;

FIG. 7 is a block diagram showing a detailed construction of an arraysensor 15, A selector circuit 13, a synchronous wave detection circuit17 and a main control system 30 in FIG. 4;

FIGS. 8(a) and 8(b) are respective explanatory views of offset valuesintroduced in this embodiment;

FIG. 9 is a block diagram showing a structural example of the correctionvalue decision part 30E in FIG. 7;

FIGS. 10(a) and 10(b) are respective views showing relations of adetection output signal FS and a signal KS;

FIG. 11 is a flow chart showing one example of the focus point detectionoperation and the exposure operation in the embodiment;

FIG. 12 is a flow chart showing a modified example of the steps 111 and112 in FIG. 11;

FIG. 13 is a view showing a relation between the detection output signalFS and the position in a Z-axis direction;

FIG. 14 is an enlarged plan view showing a case in which slit images arerespectively projected onto the measurement points which distributetwo-dimensionally in the shot area on a wafer; and

FIG. 15(a) is a view showing-the wavelength characteristics of theillumination light used in the AF system in a modified embodimentaccording to the present invention; FIG. 15(b) is a view showing thetransmittance distribution of the optical filter plate 60 used in themodified embodiment; FIG. 15(c) is a view showing the wavelengthcharacteristics of the light beam received by the array sensor 15, andthe wavelength sensitive characteristics of the array sensor 15 and FIG.15(d) is a view showing the wavelength characteristics of thephotoelectric transfer signal outputted from the array sensor 15.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Hereunder, a detailed description will be given of one embodiment of aprojection exposure apparatus according to the present invention withreference to the drawings.

FIG. 1 is a view showing a focus detecting system of the TTL (throughthe lens) type for detecting a best focus plane (image plane) of aprojection optical system in the projection exposure apparatus of thisembodiment. The focus detecting system of the UTT type in FIG. 1 isdisclosed in detail in U.S. Pat. No. 5,241,188. In FIG. 1, a reticle R,of which the undersurface is formed with a pattern area PA of a circuitfor manufacturing a real device, is held by a reticle holder, not shown.An optical axis AX of a projection optical system PL, which is shown ina model wherein a front group and a rear group are separated and adiaphragm surface (pupil surface) EP is put therebetween, passes throughthe center of the reticle R, that is, the center of the pattern area PA,in perpendicular to the reticle pattern surface. It is presumed that aZ-axis is parallel to the optical axis AX, a X-axis is parallel with thepaper surface of FIG. 1 in the plane perpendicular to the Z-axis, and aY-axis is perpendicular to the paper surface of FIG. 1.

Under the projection optical system PL, a Z-leveling stage 20 forholding a wafer W coated with photoresist is placed on a XY stage. TheZ-leveling stage 20 moves the wafer W by a micro value (for example,within ±100 μm) in the direction along the optical axis AX to performfocusing, and controls the inclination angle of the wafer W to performleveling. The XY stage 21 moves the wafer W two-dimensionally in theXY-plane perpendicular to the optical axis AX, and the coordinates ofthe XY stage 21 in the X-direction and in the Y-direction are measuredat all times by means of a laser interferometer, not shown.

Further, a reference mark plate FM is fixed on the upper surface of theZ-leveling stage 20 at substantially the same level position as thesurface of the wafer W. As shown in FIG. 2(a), the reference mark plateFM is provided with a slit mark ISy having a plurality oflight-transmissible slits extending in the X-direction and arranged inthe Y-direction at a constant pitch, a slit mark ISx having a pluralityof light-transmissible slits extending in the Y-direction and arrangedin the Y-direction at a constant pitch, and a slit mark ISa extending inthe direction including by 45° relative to X- and Y-directions,respectively. The entire surface of the quarts reference mark plate FMis evaporated with a chrome layer (shading layer) and is curved so as toform the slit marks ISx, ISy and ISa as transparent parts.

Referring to FIG. 1, a mirror M1 an objective lens 50 and an emissionend of an optical fiber are placed under the reference mark plate FM(inside of the Z-leveling stage 20). An illumination light from theemission end of an optical fiber 51 is converged by the objective lens50 and illuminates the slit marks ISx, ISy and ISa on the reference markplate FM from the back. A beam splitter 52 is provided near the entranceend side of the optical fiber 51, and an exposure illumination light IEis introduced to the optical fiber 51 through a lens system. Theilluminating light IE is desirable to be obtained from a light source (amercury lamp, and excimer laser light source or the like) forilluminating the retile A, however, another dedicated light source maybe prepared. However, when another light source is used, theallumination light must have the same wavelength as the exposureillumination light or must have the very near wavelength.

The illuminating condition of the reference mark plate FM by theobjective lens 50 is equalized with the illuminating condition in theprojection optical system PL as much as possible when the pattern isprojected. That is, the numerical aperture (N.A.) of the illuminationlight at the image side of the projection optical system PLsubstantially coincides with the numerical aperture (N.A.) of theillumination light from the objective lens 50 to the reference markplate, FM. Now, with this arrangement, when the illumination light IE isintroduced into the optical fiber 51, an image light beam entering intothe projection optical system PL is produced from the slit marks ISx,ISy and ISa on the reference mark plate FM. In FIG. 1, the position ofthe Z-leveling stage 20 in the direction along the optical axis AX isset such that the surface of the reference mark plate FM is positionedslightly under the best image plane Fo (the conjugate surface with thereticle) of the projection optical system PL. In this case, an imagelight beam L1 emitted from one point on the reference mark plate FM,passes through the center of the pupil surface EP in the projectionoptical system PL, and converges on the surface Fr slightly shiftedunder the pattern surface of the reticle R and then diverges,thereafter, it reflects on the pattern surface of the reticle R andreturns along the original optical path. The surface Fr is positioned soas to be optically conjugate with the reference mark plate FM withrespect to the projection optical system PL. When the projection opticalsystem PL is a double telecentric system, the image light beam on thereference mark plate FM from the slits mark ISx, ISy and ISa isreflected regularly and returns so as to coincide with the slit marksISx, ISy and ISa.

However, as shown in FIG. 1, when the reference mark plate FM isdisplaced from the image plane Fo, fade reflected images of therespective slit mark ISx, ISy and ISa are formed on the reference markplate FM. On the other hand, when the reference mark plate FM coincideswith the image plane Fo, the surface Fr also coincides with the patternsurface of the reticle. Therefore, sharp reflected images of therespective slit marks ISx, ISy and ISa are formed on the reference markplate FM so as to coincide with the respective marks. FIG. 2(b) showstypically the relation between the slit mark ISx when the reference markplate FM defocuses and the reflected image IMx. In the doubletelecentric projection optical system PL, the reflected image IMx isprojected on the slit mark ISx which is its source, like this. When thereference mark plate FM is defocused, the reflected image IMx becomeslarger that the slit mark ISx in the geometry and the illuminance perunit area is reduced.

Therefore, the light beam of the image portions which are not shaded bythe original slit marks ISx, ISy and ISa among the reflected imagesformed on the reference mark plate FM, is introduced into the opticalfiber 51 through the mirror M1 and the objective lens 50 and the lightbeam emitted from the optical fiber 51 is received by a photoelectricsensor 55 through the beam splitter 52 and the lens system 54. Thelight-receiving surface of the photo-electric sensor 55 is placed at aposition that is conjugate with the pupil surface (Fouriertransformation surface) EP of the projection optical system PL. In FIG.1 a contrast signal can be obtained for determining the imaged surfaceof the projection optical system PL only by moving the Z-leveling stage20 upwardly (in the Z-direction).

FIGS. 3(a) and 3(b) show the respective signal level characteristics ofthe output signal KS of the photoelectric sensor 55, and the horizontalaxis indicates the position of the Z-leveling stage 20 in theZ-direction, that is, the height of the reference mark plate FM in thedirection along the optical axis AX. FIG. 3(a) show the signal levelwhen the slit marks ISx, ISy and ISa are projected onto the chromeportion in the pattern surface on the reticle R, and FIG. 3(b) shows thesignal level when these slit marks are projected onto the glass portion(transparent portion) in the pattern surface. Generally, the chromeportion of the reticle is evaporated to a glass (quartz) plate with thethickness of about 0.3-0.5 μm, thus, the reflectance of the chromeportion is much larger than that of the glass portion. However, sincethe reflectance of the glass portion does not become to zero, the signallevel is very low as shown in FIG. 3(b) but can be detected at the glassportion. Further, a reticle for manufacturing a real device has a highdensity pattern in general, so that there may be little probability thatthe projected images of all of the slit marks ISx, ISy and ISa areformed on the glass portion (transparent portion) in the reticlepattern.

In any case, when the surface of the reference mark plate FM is moved inthe direction along the optical axis AX so as to across the best imageplane Fo, the level of the output signal KS becomes the maximum value atthe position Zo in the Z-direction. Therefore, the position of the bestimage plane Fo can be obtained by measuring the position of theZ-leveling stage 20 in the Z-direction and the output signal KS at thesame time, and detecting the position in the Z-direction when the levelof the output signal KS reaches the maximum. Moreover, with thisdetecting method, the best imaged surface Fo can be detected at anyposition on the reticle R. Therefore, whenever the reticle R is set at aside of the object plane of the projection optical system PL, theabsolute focus position (the best image plane Fo) can be measured at anypositions in the projection visual field of the projection opticalsystem PL. And, as described above, the chrome layer of the reticle Rhas the thickness of 0.3-0.5 μm, and when the projection magnificationof the projection optical system PL is set at ⅕ (reduction), thedetected error of the best image plane Fo caused by this thickness is(0.3-0.5)×(⅕)², that is, 0.012-0.02 μm, this error can be neglected.

Next, with reference to FIG. 4, an explanation will be given to anoblique incidence type AF system (a focus position detecting system) ofthis embodiment, but a multipoint AF type will be used in thisexplanation. The multipoint AF system is provided with measurementpoints for measuring a positional offset of the wafer-W in the directionalong the optical-axis (that is, focus dislocation) at plural points inthe projection visual field of the projection optical system PL. In FIG.4, an illumination light IL to which is the photoresist on the wafer Wis non-sensitive, is irradiated from a light source such as a halogenlamp, not shown, and illuminates a slit plate 1. Then, the light passedthrough a slit of the slit plate 1, illuminates obliquely the wafer Wthrough a lens system 2, a mirror A, a diaphragm 4, a projectionobjective lens 5 and a mirror 6. In this case, when the surface of thewafer W is in the best image plane Fo, the image of the slit in the slitplate 1 is imaged on the surface of the wafer W by means of the lenssystem 2 and the objective lens 5. And, the angle between the opticalaxis of the objective lens 5 and the surface of the wafer is set at5-12° and the center of the slit image of the slit plate 1 is positionedat the point where the optical axis AX of the projection optical systemPL crosses with the wafer W.

Now, the light beam of the slit image, which is reflected by the wafer,form again a slit image on a light-receiving slit plate 14 through amirror 7, a light-receiving objective lens 8, a lens system 9, anoscillating mirror 10 and a plane parallel plate (plane parallel) 12.The oscillating mirror 10 oscillates by a minute amplitude the slitimage on the light-receiving slit plate 14 in the directionperpendicular to the longitudinal direction of the slit image. On theother hand, the plane parallel 12 shifts the relation between the sliton the slit plate 14 and the center of oscillation of the slit imageformed by the reflected light from the wafer W in the directionperpendicular to the longitudinal direction of the slit. The oscillatingmirror 10 is oscillated by the mirror drive (M-DRV) 11 which is drivenby the driving signal from the oscillator (OSC) 16.

In this way, if the slit image oscillates on the light-receiving slitplate 14, the light beam which have passed through the slit plate 14 isreceived by an array sensor 15. The array sensor 15 is divided along thelongitudinal direction the slit in the slit plate 14 into a plurality ofsmall areas and individual receiving picture elements are disposed inthe respective small areas. A silicon photodiode, a phototransistor orthe like is used as a photoelectric transfer element in the arraysensor. The signals from the respective receiving picture elements inthe array sensor 15 are selected or grouped through the selector circuit13 and then supplied to a synchronous detection circuit (PSD) 17. Thesynchronous detection circuit 17 is supplied with an AC signal which hasa same phase as the driving signal from the oscillator 16, and thesignals from the receiving picture elements are rectified synchronouslybased on the phase of the AC signal.

In this case, the synchronous detection circuit 17 is provided with aplurality of detecting circuits for synchronously and individuallydetecting the respective output signals of the plurality of receivingpicture elements which are selected among the array sensor 15, and therespective detection output signals FS are supplied to the main controlunit (MCU) 30. The respective detention output signals FS are referredto as S curve signals, and become the zero level when the center of theslit in the light-receiving slit plate 14 coincides with the center ofoscillation of the reflected slit image from the wafer W, the positivelevel when the wafer W shifts upper than that state and the negativelevel when the wafer W shifts lower. Therefore, when the detectionoutput signal FS becomes the zero level, the vertical position of theexposure surface (for example, the surface) on the wafer W is detectedas a focused point. However, it is not assured that in this obliqueincidence type, the vertical position of the wafer W at the focusedpoint (the detection output signal FS is the zero level) alwayscoincides with the best image plane Fo at any time. That is, the obliqueincidence type has a virtual reference plane which is determined by thesystem itself, and when the virtual reference plane coincides with theexposure surface on the wafer W the detection output signal FS from thesynchronous detection circuit 17 reaches the zero level. The virtualreference plane is set so as to be coincided with the best image planeFo when an apparatus is manufactured, but this coincidence is notguaranteed to be maintained for a long period. Therefore, the best imageplane Fo is detected, for example, by means of the focus detectingsystem in FIG. 1, and the plane parallel 12 in FIG. 4 is inclined bycontrolling with the main control unit 30 based on this detected bestimage plane Fo to shift the virtual reference plane in the directionalong the optical axis AM, whereby the virtual reference plane can becoincided with the best image plane Fo (or, the positional relation canbe determined). A calibration action in this oblique incidence type AFsystem is disclosed in detail in U.S. patent application Ser. No.113,815 (Aug. 31, 1993) assigned to the assignee of this applicationand, therefore, the detailed explanation thereof will be omitted.

The main control unit 30 receives the output signal KS from thephotoelectric sensor 55 in FIG. 1 and functions to calibrate themultipoint AF system of the oblique incidence type, output a commandsignal DS to the drive (Z-DRV) 18 for driving the drive motor 19 of theZ-leveling stage 20 based on the respective detection output signals FSin the multipoint AF system and control the drive (the motor and thecontrol circuit thereof are included) 22 for driving the XY stage 21.

FIG. 5 is a view showing positional relation between the projectionvisual field If of the projection optical system PL and the slit imageST projected from the multi-point AF system onto the surface of thewafer W. The projection visual field If is generally circular and theshot area SA to which the pattern image in the pattern area PA of thereticle R is projected in a rectangle included in the circle. The slitimage ST is projected onto the wafer W with its longitudinal axis beinginclined by 45° with respect to the X-axis and the Y-axis, which aremoving coordinate axes of the XY stage 21. The projection of the opticalaxes AFx of the projection objective lens 5 and the receiving objectivelens 8 to the wafer extends in the direction perpendicular to the slitimage ST. Moreover, the center of the slit image ST is set so as tosubstantially coincide with the optical axis AX. With this arrangement,the slit image ST is set so as to extend in the shot area GA as long aspossible.

Generally, a circuit pattern, which has irregularities formed by anexposure step and the like till then, is formed in the shot area SA. Inthis case, as many processes for manufacturing a device are practicedrepeatedly, variations in the irregularity will increase, and theirregularity will increase in the longitudinal direction of the slitimage ST. Particularly, in the case where a plurality of chip patternsare arranged in one shot, scribe lines for separating the respectivechip patterns extending in the X-direction or the Y-direction areformed, so that differences in level more than 2 μm are created in anextreme case between points on the scribe lines and points on the chippatterns. The position where the scribe lines are formed in the slitimage ST is known previously by the shot array in the design, the chipsize in the shot and the like, so that it is possible to determine thata reflected light from any portion of the slit image ST in thelongitudinal direction is either of the reflected light from the circuitpattern and the reflected right from the scribe line.

FIG. 6 shows a state in which the light-receiving slit plate 14 and thearray sensor 15 are separated. In FIG. 6, the slit plate 14 is depositedwith a chrome layer (shading layer) on the entire surface of the glasssubstrate plate, and a transparent slit is formed at one portion thereofby etching. The slit plate 14 is fixed on a holding frame 14A, and theholding frame 14A is fixed on a print substrate 15A made of such asceramics for holding the array sensor 15 by means of screws, not shown.With this arrangement, the slit of the slit plate 14 is arranged so thatit becomes parallel to the one-dimensional array of the receivingpicture elements in the array sensor 15 and contacts with thelight-receiving picture elements. It is preferable that the slit plate14 and the array sensor 15 are approached or contacted as closely aspossible, but, and imaging lens system may be provided between the slitplate 14 and the array sensor 15, whereby the slit plate 14 and he arraysensor 15 may be optically conjugated with each other. Incidentally, thelength of the slit image ST on the wafer shown in FIG. 5 varies inaccordance with the diameter of the projection visual field If, but, ifthe magnification of the projection optical system PL is ⅕ (reduction)and the diameter of the projection visual field If is about 32 mm; thelength is preferred to be between 1 and ⅓ times as large as the diameterof the projection visual field If.

FIG. 7 shows one example of a concrete circuit construction includingthe array sensor 15, the selector circuit 13, the synchronous detectioncircuit 17 and the main control unit 30. In FIG. 7, the selector circuit13 is composed of five selector circuit sections 13A-13E, and thesynchronous detection circuit 17 is composed of five synchronousdetection circuit sections 17A-17E. A receiving picture element in thearray sensor 15 is divided into five groups Ga to Ge, and one receivingpicture element is selected from every group by the selector circuit 13.In this case, the groups Ga-Ge detect respectively the slit imagesbefore and behind five measurement points P1-P5 along the slit image STin FIG. 5. Further, in one example, in the selector circuit selections13A-13E, detected signals of the receiving picture elements whichreceive the slit images on the measurement points P1-P5 are selected.

Concretely, in FIG. 7, the group Ga of the receiving picture elements inthe array sensor 15 includes therein a plurality of receiving pictureelements, the receiving picture element, which detects the image on themeasurement point P1, is selected by means of the selector circuitsection 13A among the receiving picture elements, and then the outputsignal of the receiving picture element is supplied to the synchronousdetection circuit 17A. In addition, the selector circuit section 13Afunctions to select any one of receiving picture elements in the groupGa to send the output signal thereof to the synchronous detectioncircuit section 17A and arbitrarily select adjacent two or threereceiving picture elements to send a signal, to which these outputsignals are added, to the synchronous detection circuit section 17A.Similarly, the output signals from the receiving picture elements in thegroups Gb-Ge are selected in the selector circuit sections 13B-13E,respectively, and the selected output signals are supplied to thesynchronous detection circuit sections 17B-17E, respectively.

The synchronous detection circuits 17A-17E receive the respectivefundamental wave alternating signals from the oscillator 16 and outputthe detection output signals FSa-FSe, respectively. These detectionoutput signals FSa-FSe are respectively converted to digital data bymeans of an analog/digital converter (ADC) 30A in the main control unit30 and then supplied to a correction operation part 30B and a deviationdetecting part 30C. The correction operation part 30B also receives adata concerning the process structure of the wafer (including the dataregarding the distribution of the irregularity on the exposure surfaceand the difference in level of the irregularity) from an exposureprocess data memory 30F, and an offset value for calibrating the signalfrom a memory 30D. Then, the correction operation part 30B calculatesdetection output values corresponding to target positions in theZ-direction at the respective measurement points on the wafer based on,for example, five detection output signal values, that is, the focusdisplacement values at five points on the wafer, the data concerning theprocess structure and the like, and supplies the values to the deviationdetecting part 30C. The deviation detecting part 30C detects thedeviation between the output value from the correction operation part30B and the detection output value from the ADC 30A, and then supplies acommand signal DS to the drive 18 shown in FIG. 14 so as to decrease thedeviation.

Concretely, for example, the deviation detecting part 30C controls thedrive 18 so as to minimize the square sum of the deviation between thedetection output signal as a target from the correction calculating part30B and the detection output signals FSa-FSe, that is, by the leastsquare method. With this arrangement, the position in the Z-direction ofthe Z-leveling stage 20 and the inclination thereof are controlled andan average surface of the measurement points P1-P5 in FIG. 5 is focusedso as to coincide with the image plane of the projection optical systemPL.

Incidentally, in FIG. 5, since the measurement points P1-P5 are arrangedon one line, the inclination to be controlled is only the inclination ofthe axis which is the straight line perpendicular to the slit image STon the surface of the wafer W. To control the inclination about twoorthogonal axes on the wafer W, the measurement points P1-P5 are arrayedtwo-dimensionally (for example, a plurality of pattern images arearranged in parallel, or formed so as to cross one another), or the shotarea SA on the wafer W is scanned in a predetermined direction withrespect to the slit image and then the distribution in the level of theall shot area SA may be measured.

In FIG. 7, the offset value, which is previously memorized in the memory30D, is measured and calculated by a calibration value determining part30E. The calibration value determining part 30E obtains the deviationbetween the virtual reference plane in the multipoint AF system and thebest focus plane Fo as a deviation voltage from the zero-level of thedetected output based on five detection output signals FSa-FSe and theoutput signal from the photo-electric sensor 55. The calibrationdetermining part 30E includes an analog-digital converter fordigital-sampling the respective level of five detection outputs and thesignal KS (See FIG. 3) at the same time, a waveform memory and the like.

Referring to FIG. 9, an explanation will be given to an embodiment ofthe calibration value determining part 30E. First, the output signal KSfrom the photoelectric sensor 55 in an absolute focus detecting systemof the TTL (through the lens) type is inputted to an analog-digitalconverter (ADC) 300, and then converted to a digital value correspondingto the signal level so as to be memorized in a RAM 301 as a memory. Theaddressing of the RAM 301 is performed by a counter 304, and both of thecounting of the counter 304 and the conversion timing of the ADC 300 aresynchronous with the clock pulse from the clock generator (CLK) 303.Similarly, one of the five detection output signals FSa-FSe is suppliedto an ADC 305 through a selecting switch 308, and the digital valueconverted therein is memorized in a RAM 306, in which the addressing isperformed by a counter 307. Therefore, the waveform of the output signaland one selected, detection output signal, which vary with time, arereceived in the RAM units 301 and 306. The waveforms in the RAMs 301 and306 are used as processing data in a processing part 310 when asmoothening, a detection of a maximum value and the like are performed.

Further, the processing part 310 outputs a signal for controlling theuniform movement of the Z-leveling stage 20 in the Z-direction to thedrive 18 so as to take the signal waveforms in the RAMs 301 and 306, andoutputs the drive signals to the drive 22 for the XY stage shown in FIG.4 for moving the centers of the slit marks ISx, ISy and ISa in FIG. 2(a)to the respective measuring points of the multipoint AF system.

FIG. 10(a) shows a waveform of variation characteristics in one detectoutput signal FS. The waveform corresponds to a waveform data which isstored in the RAM 306 when the Z-leveling stage 20 moves at a constantspeed in the Z-direction within a limited area including the best focusplane. FIG. 10(b) shows the waveform of the signal KS which is stored inthe RAM 301 at that time. The synchronous direction signal becomes asubstantial point symmetric waveform with respect to the zero point and,therefore, a negative level data smaller than the zero point isanalog-digitally converted by taking the negative level intoconsideration.

In the RAM 301 in FIG. 9, the waveform at the maximum value of thesignal KS shown in FIG. 10(b) is stored in the address corresponding tothe time t, so that the processing part 310 analyzes the waveform andobtains the time T₁ at which the maximum point can be obtained. Then,the processing part 310 obtains an address point which corresponds tothe time T₁ in the RAM 306 and obtains a level ΔFS of the detectionoutput signal stored in the address point. The level ΔFS is an offsetvoltage from the zero point of the detection output signal FS. At themeasurement point in the multipoint AF system which generates thedetection output as shown in FIG. 10(a), when the wafer surface at themeasurement point is moved in the Z-direction in such a manner that thedetection output is +ΔFS, the wafer surface coincides with the bestfocus plane Fo.

Incidentally, when the circuit in FIG. 9 is used, the slit mark on thereference mark plate FM is positioned in such a manner that the centerthereof is positioned at one of the respective measurement points in theAF system, by moving the XY stage 21 shown in FIG. 4. This positioningis not necessary to be strict. The measurement point in the multipointAF system may be displaced from the center of the slit mark group byabout 100 μm in the X-direction and the Y-direction. Therefore, when themeasurement points in the AF system, that is, the measurement pointsP1-P5 in the slit image ST shown in FIG. 5 are determined, the positionof the slit mark group is moved relative to these measurement pointswithin a range of about ±100 μm in the X-direction and the Y-directionand is tilted in the Z-direction, whereby the coordinate position atwhich the peak of the signal KS becomes large to some extent may beobtained. This is to avoid the disadvantage as much as possible that allof the slit mark group coincide with the transparent portion of thereticle R (the SN ratio of the signal KS lowers), although theprobability is very low. However, when the calibration is performed at ahigh speed, the offset value ΔFS can be obtained with similar accuracythough the coordinate position at which the peak of the signal becomeslarge is not searched. The offset values can be obtained for therespective measurement points P1-P5.

In this way, the values of the detection output signals FSa-FSe when therespective measurement points P1-P5 coincide with the position of thebest image plane in the Z-direction by the projection optical system PL,that is, the offset values BFa-BFe at the best image plane can beobtained. In FIG. 5, when the short area AS is scanned, for example, inthe X-direction with respect to the slit image ST so as to obtain thedetection output signals at the measurement points which are distributedon the entire surface of the short area SA, the offset value at eachmeasurement point is one of the offset values BFa-BFe which are obtainedas described above.

Next, an explanation will be given to an example of the focusing and theexposure action in this embodiment with reference to FIGS. 5, 8, 11 and12. In this case, it is presumed that the values of the detection outputsignals FSa-FSe when the measurement points P1-P5 in FIG. 5 arerespectively focused on the image plane of the projection optical systemPL, that is, the offset values BFa-BFe of the image plane with respectto a virtual reference plane in the multipoint AF system, are measuredpreviously. If the rotation angle of the plane parallel 12 is adjusted,the offset values BFa-BFe can be made substantial 0 (zero), so that, theoffset values BFa-BFe are values near to 0. The travelling surface alongwhich the Z-leveling stage 20 moves when the XY stage 21 is driven andthe best image plane of the projection optical system PL aresubstantially parallel.

First, in the Step 101 in FIG. 11, the XY stage is driven so as to movethe center portion of the short area SA to be measured (exposed) ontothe projection area of the slit image ST from the oblique incidentmultipoint AF system as shown in FIG. 5. Then, in the step 102, theautofocus is performed at the measurement point P3, that is, the centerof the slit image ST. That is, the height of the Z-leveling stage 20 inthe Z-direction is adjusted in such a manner that the detection outputsignal FSc corresponding to the measurement point P3 becomes the offsetvalue BFc of the best image plane, and then the Z-leveling stage 20 islocked in this state. Therefore, after that, the level and theinclination of the Z-leveling stage 20 are constant until themeasurement is finished. The reason why autofocus is performed once, isto prevent the distribution of irregularities in the shot area SA fromgetting out of the detection area in the multipoint AF system.

However, in this embodiment, when there is a plane to be a referenceplane inside or in the vicinity of the shot area SA, the autofocusingmay be performed on this plane, instead of autofocusing at themeasurement point P3 at the center of the slit image ST in Step 102. Inthis case, there is no need that a measurement point is P3, and ameasurement point which is nearest to this plane may be selected.Further, a measurement point to be autofocused may be determined usingthe exposure process data. In short, the measurement point is notnecessary to be P3, but may be any point in the scanning area, if thedisplacement value at the focus position, which is detected by themultipoint AF system, is not off from the detection area (which isdetermined by the S curve), when the wafer is scanned by the slit imageST by means of the multipoint AF system.

Next, in Step 103, the XY stage 21 is driven, so that the shot area SAis moved in the X-position to the measurement starting position SBbefore the slit image ST with respect to the direction of scanning.Then, in Step 104, the XY stage 21 is driven and the shot area Sa isscanned in the X-direction with respect to the slit image ST, and thenthe respective detection output signals FSa-FSe are stored in the memoryin the correction operation art 30B. In this case, since the coordinateof the XY stage 21 is measured by a laser interferometer, the detectionoutput signals FSa-FSe are sequentially stored at the addresses, whichcorrespond to the coordinates measured by the laser interferometer, inthe memory. Thereafter, in Step 105, the process difference in level inthe shot area SA are classified based on the obtained detection outputsignals FSa-FSe (respective time series signals).

Concretely, FIG. 8(a) shows a section in the shot area SA on the waferW, and the measurement points P1-P5 are set at this section. Actually, aphotoresist is coated on the wafer W, however, the photoresist isomitted. In FIG. 8(a), when the respective measurement points P1-P5 arereached to the virtual reference plane 41 in the multipoint AF system,the corresponding detection output signals become zero respectively.And, it is assumed that the best image plane 42 of the projectionoptical system PL is off from the virtual reference plane 41 in someextent. In this case, when the detection output signals, which can beobtained at the respective measurement points P1-P5, are FSa-FSe, thevalues of these detection output signals correspond to theirregularities.

For example, when the measurement points P1, P2 are positioned on thepattern area 40A of the convex portion on the wafer W, the measurementpoint P3 is positioned on the pattern area 40B of the concave portionand the measurement positions P4, P5 are positioned on the pattern area40C of the convex portion, the value of the detection output signal FScat the measurement point P3 becomes minimum. With this feature, thecorrection operation part 30B in FIG. 7 in this embodiment obtains thedifferences in the detection output signals corresponding to theadjacent measurement points to thereby obtain the distribution of theconcave and convex portions in the shot area. The correction operationpart 30B is supplied with the data concerning to the process structurefrom the exposure process data memory part 30F, so that the correctionoperation part 30B can distinguish the pattern areas 40A to 40C in whichthe measurement points P1-P5 are positioned by comparing distribution ofthe concave and convex portions, which is obtained as described above,with the process structure.

With this arrangement, it can be determined that the respective patternareas 40A-40C belong to any of a memory cell part, a peripheral circuitpart (logic part) a scribe line or the like. The correction operationpart 30B can recognize the difference in level Z_(A) of the respectivepattern area 40A, 40C and the difference in level Z_(B) of the patternarea 40D by the supplied data. These differences in level are differencein heights from the portion on which there is no circuit pattern of thewafer W, and as described later, differences in these levels areimportant.

Further, the dispersion and the like of the detection output signal ineach stepped area is obtained based on information regarding differencein level, which is obtained from the difference data between saidadjacent measurement points, whereby a difference in level caused by adifference in the pattern intensity in each stepped area can be known.Therefore, stable measurement points around the measurement points P1-P5can be also obtained.

Next, in Step 106, a surface to be focused on the shot area SA isdetermined as a focusing reference plane. For example, in FIG. 8(a), itis presumed that a pattern, which has narrowest line width, is exposedon the pattern area 40B with the measurement point P3, and the patternarea 40B is a focusing reference plane. However, there is a case in thatthe broadest (large) pattern area (such as the pattern area 40A) in theshot area SA is the focusing reference plane. The focusing referenceplane may be selected and determined in accordance with the priority offocusing (which is determined based on the pattern line width, the pitchand the like) of every pattern area in the shot area.

Then, in Step 107, offset values Δa-Δe for the detection output signalsFa-Fe in the measurement points P1-P5 are obtained. In FIG. 8(a), if theconversion coefficient from the detection output signal to thedisplacement in the Z-direction is k, the offset value Δc for thedetection output signal Fc at the measurement point P3 on the patternarea 40B which is the focusing reference plane is zero. The offsetvalues Δa, Δb, Δd, Δe for the detection output signals Fa, Fb, Fd, Fe atthe measurement points P1, P2, P4, P5 are (Z_(A)−Z_(B))/k respectively.

Next in Step 108, the correction operation part 30B adds the offsetvalues Δa-Δe obtained in step 107 to the offset values BFa-BFe of thedetection output signals on the best image plane 42 in FIG. 8(a). Thisis equal to that the best image plane 42 shown by the solid line isconverted to the best image plane 42A including the virtual differencein level shown by the dotted line, therefore, the pattern areas 40A-40Care focused to the virtual best image plane 42A.

That is, in Step 109, the correction operation part 30B supplies thedetection output signals of the virtual best image plane 42A, i.e.,(BFa+Δa)-(BFe+Δe) to the deviation detecting part 30C. The detectionoutput signals Fa-Fe corresponding to the real pattern areas 40A-40C aresupplied to the deviation detecting part 30C in real time. Then, thedeviation detecting part 30C supplied to the drive 18 for the Z-levelingstage 20 such drive signals; as the square sum of the deviation betweenthe offset values (BFa+Δa)-(BFe+Δe) and the detection output signalsFa-Fe obtained by using, for example, a least square method, becomeminimum. With this arrangement, as shown in FIG. 8(b), the pattern area40B which is the focusing reference plane coincides with the real bestimage plane 42. Then, in Step 110, the exposure operation is performedto exposure the pattern with the narrowest line width with highresolution.

At that time, the pattern areas 40A, 40C other than the pattern area 40Bare set within the focal depth of the projection optical system.However, in said Step 107, if the offset values Δa-Δe exceed the focaldepth, for example, the focusing reference plane may be shifted in theZ-direction apparently in such a manner that the pattern areas 40A, 40Ccome into the focal depth by weighing the offset values Δa-Δe. This isavailable when the entire surface of the shot area is within the focaldepth. Further, simply the focusing reference plane (pattern area 40B)may be shifted so that the pattern areas 40A, 40C are within the widthof the focal depth.

Incidentally, as shown in FIG. 7, since this embodiment adopts a methodof comparing a desired value with a real detection output signal in thedeviation detecting part 30C, the offset values Δa-Δe of the differencesin level are added to the best image plane 42 which is the desiredvalue. However, in FIG. 7, when the method of offset-correcting the realdetection output signal and supplying to the deviation detecting part30C is adopted, the offset values Δa-Δe may be subtracted from the realdetection output signals.

Further, in actual, the detection output signals at the measurementpoints, which are distributed over the entire surface of the shot areaSA in FIG. 5, are obtained, whereby the irregular distribution ofconcave and convex portions on the entire surface of the shot area SA isdiscriminated. However, in FIG. 7, the detection output signals whichare supplied from the ADC 30A in real time, are only the detectionoutput signals at the five points on the slit image ST in FIG. 5. Theinclination around the axis parallel with the slit image ST is notcorrected only by using the data on the slit image ST. Then, theinclination of the wafer W around the axis parallel with the slit ST iscorrected by such as an open loop. That is, the shot area SA is scannedwith respect to the slit image ST, whereby, as shown in FIG. 8(a), thedetection output signal of the virtual best image plane 42A and thedetection output signal in the actual pattern area are obtained. Then,the relation between the control value for the drive 18 and theinclination of the Z-leveling stage 20 are obtained previously, and theinclination of the Z-leveling stage 20 is controlled so as to eliminatethe difference between the detection output signal in the actual patternarea and the detection output signal fo the virtual best image plane42A. Therefore, with the open loop control, the pattern area, on whichthe pattern with the narrowest line width distributing over the entiresurface of the shot area SA in FIG. 5 is exposed, is generally focusedonto the best image plane in the projection optical system PL.

Moreover, at Step 104 in FIG. 11, as shown as Step 104A, the detectionoutput signals Fa-Fe may be stored in the memory whenever the XY stage21 stops after stepping at a constant pitch in the X-direction, that is,at the constant pitch. With this method, the air fluctuation influenceby the movement of the XY stage 21 can be reduced.

Instead of the operation in Steps 101-102, Steps 111-112 in FIG. 12 maybe available. That is, in Step 111, first, the XY stage 21 is driven soas to move the shot area SA to the measurement starting position SB asshown in FIG. 5. And then, in step 112, the autofocusing is performed atthe central measurement point P3 of the slit image ST in FIG. 5, and thefocus position of the Z-leveling stage 20 is locked. Then, at Step 104or 104A in FIG. 11, the detection output signals are sampled over theentire surface in the shot area SA. The processes after that are same asthe operation in FIG. 11. In a sequence shown in FIG. 12, there is noloss for the action of the wafer stage 21, so that the measurement canbe performed efficiently.

Further, in this embodiment, the travelling surface along which theZ-leveling stage 20 moves and the best image plane in the projectionoptical system PL are substantially parallel when the XY stage 21 isdriven in FIG. 4. However, when the travelling surface along which theZ-leveling stage 20 moves and the image plane in the projection opticalsystem PL are not parallel, the following correction operation isnecessary. That is, the deviation (an inclination of an image plane, acurvature of an image plane and the like) between the travelling surfaceof the Z-leveling stage 20 and the image plane of the projection opticalsystem PL while the XY stage 21 is driven is stored in the memory of thecorrection operation part 30B as a device constant. In this case, thedetection output signal, which has been obtained by the method of Step104 in FIG. 11, shows the measured result when the travelling surface ofthe Z-leveling stage 20 is a reference and, therefore, only thedeviation value from the image plane, which has been memorized as thedevice constant, is added to the measured result.

And, in this embodiment, as shown in FIG. 5, the distribution of concaveand convex portions is obtained in one shot area on the wafer W,however, the same measurements are performed in different some (morethan one) shot areas on the wafer W and the obtained detection outputsignal are averaged, and then the distribution of concave and convexportions in the respective shot areas on the wafer may be obtained bycomparing the averaged result with the process structure. With thisarrangement, the effects such as the uneven coating can be reduced.

Next, in this embodiment, as shown in FIG. 5, the position in theZ-direction is detected in the predetermined shot area SA on the waferW, however, the position in the Z-direction may be detected at a pitchwhich is 1/N (N is integer number) of the array pitch in than shot areaover the entire surface of the wafer. Concretely, if the array pitch ofthe shot area in the X-direction is Px, the distance of between adjacentfocus positional measurements is Px/N, wherein N is the integer numbergreater than 2. In this case, the detection output signal from themultipoint AF system changes repeatedly at the same cycle as the arraypitch in the shot area.

In this case, when there is a foreign material such as dust on theexposure surface on the wafer or the exposure surface is deformed suchas by camber of the wafer, the change in the output of the multipoint AFsystem in the shot area differs from that in another shot area. Thus, itis desirable for the shot area, in which the deviation from the averageof the sampled detection outputs signals at the cycle corresponding tothe array pitch in the shot area is above the predetermined threshold,to calculate separately an offset value of the detection output signalfor the focusing reference plane. A process such as an assist process(operator call) may also be performed as an alarm or an error for theshot area which subject to the effect such as by the foreign material orcamber.

Next, in this embodiment, the position in the Z-direction of theexposure surface on the wafer W (the focus position) is measured by thedetection output signal FS which varies in an S-curve shape.

The curve 44 in FIG. 13 shows one of the detection output signals. InFIG. 13, conventionally, the section, in which the curve 44 can beapproximated by the straight line 45, is used to obtain the position inthe Z-direction from the detection output signal FS. However, thismethod has a disadvantage that the position detecting area in theZ-direction is narrow. Therefore, to broaden the position detectingarea, for example, the detection output signal FS (actually, thedetection output signals FSa-FSe are measured respectively) while theZ-leveling stage 20 in FIG. 4 is moved in the Z-direction at the movingpitch ΔZ stored in the memory, that is, it is desirable that the curve44 in FIG. 13 is obtained approximately. In this case, the value of thedetection output signal FS is stored for the position in theZ-direction.

Then, when the position in the Z-direction is measured actually, if thevalue of the detection output signal FS is V_(i), the position Z_(i) inthe Z-direction can be obtained correctly from the curve 44. However, ifthe curve 44 is approximated by the straight line 45, the position inthe Z-direction is Z_(h) when the detection output signal is V_(i), sothat an error occurs.

Next, in the above embodiment, the inclination of the Z-leveling stage20 is controlled based on the actually measured result. However, theinclination of the image plane of the projection optical system PLrelative to the surfaces of the XY stage 21 along which the Z-levelingstage moves is already known and, therefore, the inclination may becontrolled previously by the Z-leveling stage 20. With this arrangement,when the position in the Z-direction is detected by means of themultipoint AF system, the angular offset in the angle of inclination canbe reduced, so that an offset value calculated every measurement pointbecomes small. Therefore, the time for focusing can be shortened and thefocusing accuracy can be improved.

In the above embodiment, as shown in FIG. 4, with the angle ofinclination of the plane parallel 12 placed in the light-receivingsystem of the multipoint AF system, the positional relation between thevirtual reference plane and the best image plane in the multipoint AFsystem can be adjusted. This also means that the offset values whichoccur in the detection output signals FS-FSe in common can be eliminatedby the angle of inclination of the plane parallel 12.

However, when the plane parallel 12 is provided only for thelight-receiving system side, the correction volume is small, so that aplane parallel may be arranged at the light-sending system side. In thisway, an image position is corrected by two plane parallel, whereby thecorrection volume of the image position can be made large. Further, theplane parallels are arranged both of the light-sending system and thelight-receiving system so as to correct the image position, whereby thepositional displacement of a bright and dark pattern on the wafer can becorrected also.

If a plane parallel is arranged in the light-sending system in themultipoint AF system and when a common offset correction is performedusing this plane parallel, for example, in Step 109 in FIG. 11, thepositional displacement on the slit image ST on the wafer W is createdbetween the case where the detection output signal is measured in Step104 and the case where the detection output signal is measured in Step109. Then, in order to reduce the influence of the positionaldisplacement, the positional displacement value of the slit image ST onthe wafer W relative to the angle of inclination of the plane parallelis measured previously, and the offset values to be added to the offsetvalues of the best image plane for the respective measurement points maybe corrected based on the previously measured positional displacementvalue.

Next, in the above embodiment, as shown in FIG. 5, the slit image ST fordetecting the position in the Z-direction is projected obliquely in thediagonal direction relative to the shot area SA on the wafer W, and fivepoints on the slit image ST are selected as the measurement pointsP1-P5. On the other hand, as shown in FIG. 14, N (N is 25 in FIG. 14)measurement points P11, P12, . . . . P74 may be set two-dimensionally inthe X-direction and the Y-direction with a predetermined pitch, andpattern images for detecting the focus may be projected on themeasurement points, respectively. In this case, the number of receivingelements (receiving picture elements) for receiving the respectivepattern images is the same as the number of measurement points. Thus,for example, if a synchronous detection method is used, it is difficultto process the photoelectric transfer signals of the pattern images fromall measurement points in parallel. Therefore, for example, using theselector circuit sections 13A-13E shown in FIG. 7, five photoelectrictransfer signals are selected from the total photoelectric transfersignals (total number is N), and the synchronous detection may beperformed time-divisionally. With the time-division method, the circuitstructure can be made simple.

In order to detect the focus, instead of projection the slit image, suchas a grid-like fright and dark pattern with predetermined pitches may beprojected obliquely on the wafer. In this case, using the reflectedlight from the wafer, the grid-like bright and dark pattern is re-imagedon a two-dimensional image pickup element such as, for example, atwo-dimensional CCD, and the positional displacement value in the Zdirection on the exposure surface on the wafer can be obtained inaccordance with the horizontal displacement value of the re-imagedimage.

A method wherein the slit image is projected, and the position of thepattern image which is re-imaged on one-dimensional line sensor or thelike may be detected so as to obtain the positional displacement valuein the Z direction is also applicable. In this method, a plane parallelfor calibration is not necessary, and an electric offset may be used. Atleast one measurement point may be set for each of at least two patternareas (including a scribe line and the like) the levels of which aredifferent in the short area. However, for example, when a plurality ofmeasurement points are set in the respective pattern areas, and whenoffset values Δa-Δe are obtained, the plurality of measurement pointsare processed in a statistical or an averaging, or weighted-averagingmethod for every area, and when the autofocusing is performed, theobtained offset value is applied to one measurement point for every areaand the detection output signal at that measurement point may be used.In short, when there exists a plurality of measurement points in onepattern area, it is not necessary to obtain offset values for therespective measurement points and to perform autofocusing so as to fitall of the shot surfaces with the respective image planes at each of theplurality of measurement points, so that an offset value at least onemeasurement point may be obtained for every pattern area and theautofocusing may be performed using the measurement point.

Next, in the oblique incident type AF system (the focus positiondetecting system) shown in FIG. 4 of this embodiment, a light having awavelength to which the photo-resist on the wafer W is not or lesssensitive is used as the illumination light IL for detecting the focus.Further, in the photoresist, a thin film interference occurs by theincident light beam and, therefore, when the light beam is amonochromatic light, there is a case in that the intensity of thereflected light becomes very faint by the thickness of the photoresist.Then, in order to reduce the adverse effect by the thin filminterference, it is desirable to use the light beam having a band-passwidth more than 100 nm as the illumination light IL. More specifically,as an illumination light, the light beam having a wavelength width suchas 700 nm-900 nm, which has been selected from the light beamilluminated from a halogen lamp through the wavelength selecting filter,can be used. And, the illumination light having the wavelength widthabout 700 nm -900 nm from a light emitting diode may be used. Further, aplurality of monochromatic lights, which can be obtained by mixing lightbeams from a plurality of semiconductor laser elements or the like, maybe used as the illumination light IL.

However, when a light beam having predetermined wavelength width orhaving a plurality of wavelength is used as the illumination light ILand the distribution of the light intensity to the wavelength is notuniform, for example, the light intensity of a specific wavelength isstrong, there is a danger that the illuminating light. is influenced bya thin film interference effect with the specific wavelength. Thus, inorder to avoid this effect, as shown in FIG. 4, it is desirable that theoptical filter plate 60 for equalizing the distribution of thephoto-electric transfer signal for the wavelength is arranged in frontof the array sensor 15 of the AF system. In addition, the optical filterplate 60 may be placed at any position between a light source, notshown, for generating the illumination light IL and the array sensor 15.

Referring to FIG. 15, a concrete explanation will be given to oneexample of the characteristics of the optical filter plate 60. First, itis assumed that the distribution of the light intensity L_(E)(λ) for thewavelength λ of the illumination light IL is generally a V shape asshown in FIG. 15(a). In this case, the distribution of the transmittanceT (λ) for the wavelength λ of the optical filter plate 60 is set in asubstantial inverted V shape, as shown in FIG. 15(b). However, thetransmittance T (λ) is corrected in consideration of the wavelengthsensitive characteristics in the array sensor 15.

That is, it is presumed that the detection sensitivity (outputsignal/incident light intensity) PSV (λ), for the wavelength λ in thearray sensor 15 increases as the wavelength λ increases, as shown by thedotted line in FIG. 15(c). In this case, the distribution of the lightintensity L_(R)(λ) for the wavelength λ of the light beam, which isreceived by the array sensor 15 through the optical filter plate 60 isproduct of the light intensity L_(E)(λ) and the transmittance T (λ), sothat the distribution of the transmittance T (λ) is set in such a mannerthat the distribution of the light intensity L_(R)(λ) slightly decreasesas the wavelength λ increases, as shown by the solid line in FIG. 15(c).In this case, the photoelectric transfer signal SR (λ) outputted fromthe array sensor 15 for the light beam having the wavelength λ isproduct of the detection sensitivity PSV (λ) and the light intensityL_(R)(λ), so that it becomes almost even relative to change in thewavelength λ. With this arrangement, the adverse effect of the thin filminterference on the photoresist can be reduced, so that the differencein level on the wafer surface can be measured stably.

Moreover, the present invention should not be limited to this, so thatvarious structures can be applied within a limitation which is notbeyond the points of the present invention.

According to the present invention, arithmetic unit is provided forobtaining offset values for fitting the focusing reference plane on thesubstrate to the image plane by the projection optical system for eachof a plurality of measurement points separately, based on the respectivedetected signals of the photoelectric detecting device corresponding tothe plurality of measurement points and the process structure of theexposure surface of the substrate, therefore, there is an advantage thatthe respective exposure areas (shot areas) can be fitted to the imageplanes of the projection optical system and exposed under optimumcondition irrespective of an irregular condition in the respective shotareas on the substrate.

Moreover, in the case where the substrate stage is driven so as to movethe substrate with the image of the focus-detecting pattern beingprojected from the focusing projection optical system onto the exposurearea of the projective optical system, whereby the detection signals ofthe photoelectric detecting devices corresponding to the plurality ofmeasurement points which are distributed over the entire surface in theexposure area are obtained respectively; and in the case where thearithmetic unit obtaining offset value for fitting the focusingreference plane on the substrate to the image plane by the projectiveoptical system for each of the plurality of measurement pointsseparately, based on the detection signals of the photoelectricdetecting device at the measurement points which are distributed overthe entire surface and the process structure of the exposure on thesubstrate the irregular condition of the entire surface in the exposurearea on the substrate can be measured rapidly by using an optical systemfor focus-detecting having simple structure. Therefore, there is anadvantage that in an optimum condition the entire surface of theexposure area can be fitted to the image plane by the projection opticalsystem so as to be exposed. And, although the surface of a holding tool(such as a wafer holder) for the substrate is not flat, there is acamber in the substrate or there is foreign materials and the likebetween the substrate and the holding tool, a focusing error caused bythe above matters can be prevented. That is, the entire surface of theexposure area and the image plane can be fitted to each other or set inthe focal depth.

Further, in the case where a light beam having a band-pass width morethan 100 nm is used when the pattern image for focus-detecting isprojected from the projection optical system onto the exposure area bythe projection optical system, there is an advantage that an adverseeffect of the thin film interference in the photosensitive material(photoresist and the like) on the substrate can be reduced. There is acase that the light beam is diverged or diffracted by the irregular edgeportion and the like on the substrate, however, if a light beam havingwide band-pass is used, there is an advantage that the detection signalhaving a fine SN ratio can be obtained, even if the light beam of thespecific wavelength is feeble.

Moreover, when the optical filter for uniforming the wavelengthsensitive characteristics of the light beam used while the pattern imagefor focus-detecting is projected is arranged in the optical path fromthe projection optical system to the plurality of photoelectricdetecting devices, for example, in a case where the light beam with awide band-pass width is used, the distribution of intensity of thedetection signal outputted from the photoelectric detecting devicerelative to the wavelength is substantially flatten. Therefore, thedistribution in level of the exposure surface on the substrate can bemeasured correctly, without an influence of the light having apredetermined wavelength.

Next, when the arithmetic unit corrects a desirable value according tothe level of the image plane by the projection optical system based onthe offset values which are obtained separately at respective pluralmeasurement points, the closed loop control is performed so as tocoincide the corrected desirable value with the actually obtaineddetection signal, whereby the focusing and the leveling can be performedwith a high degree of accuracy.

As described above, the explanation is given to the preferredembodiment, however, needless to say that person those skilled in theart can make a modification or revision in the scope of the presentinvention based on the above disclosure.

What is claimed is:
 1. An exposure method for projecting an image of apattern on a mask onto a substrate through a projection system,comprising: detecting a positional information of said substrate in adirection along an optical axis of said projection system at a pluralityof measurement points which are set beforehand in a predeterminedpositional relation with respect to an image field of said projectionsystem; scanning said plurality of measurement points and a plurality ofshot areas on said substrate relative to each other; obtaining saidpositional information at a plurality of points in each of said shotareas in connection with relative movement between said plurality ofmeasurement points and said plurality of shot areas; and obtainingdistribution of concave and convex portions in each of said shot areas.2. An exposure method according to claim 1, wherein the processdifference in level in said shot area is classified based on saiddistribution of concave and convex portion in each of said shot areas.3. An exposure method according to claim 2, wherein, among a pluralityof stepped areas obtained by said classification, the stepped area onwhich a pattern with the narrowest line width is projected is determinedas a focusing reference plane for aligning said stepped areas of saidpattern with an image plane of said projection system.
 4. An exposuremethod according to claim 1, wherein offset information for positionalinformation obtained at said plurality of measurement points is obtainedbased on said distribution of concave and convex portion in saidplurality of shot areas.
 5. An exposure method according to claim 1,wherein an average distribution of concave and convex portion in saidshot areas is obtained by averaging said distribution of concave andconvex portion in each of said shot areas.
 6. An exposure methodaccording to claim 5, further comprising: adjusting a relative positionin a direction along an optical axis of said projection system betweensaid substrate and an imaging plane of said projection system based on adetected result of the position of said substrate in said directionobtained at said plurality of measurement points and said averageddistribution of convex and concave portion in said shot area, when saidsubstrate area is exposed.
 7. A method of manufacturing a semiconductordevice using the exposure method set forth in claim 1 comprising anexposing step of projecting an image of a pattern on a mask onto asubstrate through a projection system.
 8. A projection exposure methodfor projecting an image of a pattern on a mask onto a substrate placedon a stage through a projection optical system, comprising steps of:detecting a positional information of said substrate in a directionalong an optical axis of said projection system at a plurality ofmeasurement points which are set beforehand in a predeterminedpositional relation with respect to an image field of said projectionsystem; moving said stage to scan said plurality of measurement pointsand a shot area on said substrate relative to each other; obtaining saidpositional information at a plurality of points in said shot area inconnection with relative movement between said measurement light andsaid plurality of shot areas; and obtaining distribution of concave andconvex portions in said shot area based on said positional informationand information of a travelling surface of said stage.
 9. A projectionexposure method according to claim 8, wherein the process difference inlevel in said shot area is classified based on said distribution ofconcave and convex portion in each of said shot areas.
 10. A projectionexposure method according to clam 9, wherein, along a plurality ofstepped areas obtained by said classification, the stepped area on whicha pattern with the narrowest line width is projected is determined as afocusing reference plane for aligning said stepped areas of said patternwith an image plane of said projection system.
 11. A projection exposuremethod according to claim 8, wherein an offset information forpositional information obtained at said plurality of measurement pointsis obtained based on said distribution of concave and convex portion insaid plurality of shot areas.
 12. A projection exposure method accordingto claim 8, further comprising: adjusting a relative position in adirection along an optical axis of said projection system between saidsubstrate and an imaging plane of said projection optical system basedon a detected result of the position of said substrate in said directionobtained at said plurality of measurement points and said distributionof convex and concave portion in said shot area, when said substrate isexposed.
 13. A method for manufacturing a semiconductor device using theexposure method set forth in claim 8 comprising an exposing ofprojecting an image of a pattern on a mask onto a substrate through aprojection system.
 14. An exposure apparatus which projects an image ofa pattern on a mask onto a substrate through a projection system,comprising: a detector which detects a positional information of saidsubstrate in a direction along an optical axis of said projection systemat a plurality of measurement points which are set beforehand in apredetermined positional relation with respect to an image field of saidprojection system; a scanning system which scans said plurality ofmeasurement points and a plurality of shot areas on said substraterelative to each other; a controller which is connected to said detectorand said scanning system and obtains said positional information at aplurality of points in each of said shot areas in connection withrelative movement between said plurality of measurement points and saidplurality of shot areas; and a calculation system which is connected tosaid controller and obtains distribution of concave and convex portionsin each of said shot areas.
 15. An exposure apparatus according to claim14, wherein said calculation system classifies the process difference inlevel in said shot area based on said distribution of concave and convexportion in each of said shot areas.
 16. An exposure apparatus accordingto claim 15, wherein said calculating system determines the stepped areaon which a pattern with the narrowest line width is projected among aplurality of stepped areas obtained by classification as a focusingreference plane for aligning said image of said pattern with an imageplane of said projection system.
 17. An exposure method according toclaim 14, wherein said calculation system obtains an offset informationfor positional information obtained at said plurality of measurementpoints based on said distribution of concave and convex portion in saidplurality of shot areas.
 18. An exposure method according to claim 14,wherein said calculation system obtains an average distribution ofconcave and convex portion in said shot areas by averaging saiddistribution of concave and convex portion in each of said shot areas.19. An exposure apparatus according to claim 18, wherein said controlleradjusts a relative position in a direction along an optical axis of saidprojection system between said substrate and an imaging plane of saidprojection system based on a detected result of the position of saidsubstrate in said direction obtained at said plurality of measurementpoints and said averaged distribution of convex and concave portion insaid shot areas, when said substrate is exposed.
 20. A projectionexposure apparatus which projects an image of a pattern on a mask onto asubstrate placed on a stage through a projection optical system;comprising: a detector which detects a positional information of saidsubstrate in a direction along an optical axis of said projection systemat a plurality of measurement points which are set beforehand in apredetermined positional relation with respect to a image field of saidprojection system; a scanning system which moves said stage to scan saidplurality of measurement points and a shot area on said substraterelative to each other; a controller which is connected to said detectorand said scanning system and obtains said positional information at aplurality of points in said shot area in connection with relativemovement between said measurement light and said plurality of shotareas; and a calculating system which is connected to said controllerand obtains distribution of concave and convex portions in said shotarea based on said positional information and information of atravelling surface of said stage.
 21. A projection exposure apparatusaccording to claim 20, wherein said calculating system classifies theprocess difference in level in said shot area based on said distributionof concave and convex portion in each of said shot areas.
 22. Aprojection exposure method according to claim 21, wherein, among aplurality of stepped areas obtained by said classification, the steppedarea on which a pattern with the narrowest line width is projected isdetermined as a focusing reference plane for aligning said stepped areasof said pattern with an image plane of said projection system.
 23. Aprojection exposure method according to claim 20, wherein saidcalculating system obtains an offset information for positionalinformation obtained at said plurality of measurement points is obtainedbased on said distribution of concave and convex portion in saidplurality of shot areas.
 24. A projection exposure method according toclaim 20, wherein said controller adjusts a relative position in adirection along an optical axis of said projection system between saidsubstrate and an imaging plane of said projection optical system basedon a detected result of the position of said substrate in said directionobtained at said plurality of measurement points and said distributionof convex and concave portion in said shot area, when said substrate isexposed.